Photosynthetic parameters and primary production, with focus on large phytoplankton, in a temperate mid-shelf ecosystem

Estuarine, Coastal and Shelf Science 154 (2015) 255e263

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Photosynthetic parameters and primary production, with focus on large phytoplankton, in a temperate mid-shelf ecosystem
Anxelu G. Mora
n a, b, *, Renate Scharek b Xose a b

Red Sea Research Center, King Abdullah University of Science and Technology, 23955-6900 Thuwal, Saudi Arabia
fico de Xixo
n, Instituto Espan ~ ol de Oceanografía, Camín de L'Arbeyal s/n, E-33212 Xixo
n, Asturies, Spain Centro Oceanogra

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2014 Accepted 30 December 2014 Available online 9 January 2015

Annual variability of photosynthetic parameters and primary production (PP), with a special focus on large (i.e. >2 mm) phytoplankton was assessed by monthly photosynthesis-irradiance experiments at two depths of the southern Bay of Biscay continental shelf in 2003. Integrated chl a (22e198 mg m
2) was moderately dominated by large cells on an annual basis. The March through May dominance of diatoms was replaced by similar shares of dinoflagellates and other flagellates during the rest of the year. Variability of photosynthetic parameters was similar for total and large phytoplankton, but stratification affected the initial slope aB [0.004e0.049 mg C mg chl a
1 h
1 (mmol photons m
2 s
1)
1] and maximum B (0.1e10.7 mg C mg chl a
1 h
1) differently. P B , correlated positively with aB only photosynthetic rates Pm m B tended to respond faster to ambient irradiance than aB, which was negatively for the large fraction. Pm correlated with diatom abundance in the >2 mm fraction. Integrated PP rates were relatively low, averaging 387 (132e892) for the total and 207 (86e629) mg C m
2 d
1 for the large fraction, probably the result of inorganic nutrient limitation. Although similar mean annual contributions of large phytoplankton to total values were found for biomass and PP (~58%), water-column production to biomass ratios (2e26 mg C mg chl
1 d
1) and light utilization efficiency of the >2 mm fraction (0.09e0.84 g C g chl
1 mol photons
1 m2) were minimum during the spring bloom. Our results indicate that PP peaks in the area are not necessarily associated to maximum standing stocks. © 2015 Elsevier Ltd. All rights reserved.

Keywords: phytoplankton photosynthesis-irradiance relationships primary production: size annual variations Bay of Biscay

1. Introduction Phytoplankton is the foundation of marine food webs in pelagic waters and consequently its biomass, usually expressed as chlorophyll a concentration, is widely used as an indicator of ecosystem productivity and trophic status. However, photosynthetic carbon fixation is also dependent on light, nutrient availability and community composition among other factors, indirectly related to standing stocks. In temperate waters the predictable nature of the first two factors associated with seasonal variations in water column stability also influence largely the composition of phytoplankton dominant assemblages (e.g. Cullen et al., 2002). However, due to the transient nature of phytoplankton blooms, changes in community composition are much more difficult to predict than

* Corresponding author. Red Sea Research Center, King Abdullah University of Science and Technology, 23955-6900 Thuwal, Saudi Arabia. E-mail addresses: [email protected], [email protected], xelu.moran@gi. ieo.es (X.A.G. Mor
an). http://dx.doi.org/10.1016/j.ecss.2014.12.047 0272-7714/© 2015 Elsevier Ltd. All rights reserved.

most physico-chemical properties. A marked dominance of chain forming diatoms in the algal blooms occurring in late winter-early spring and autumn in temperate coastal waters is well documented (Ianson et al., 2001; Winder and Cloern, 2010). For the rest of the year, non-diatomaeous pico- and nanophytoplankton and occasionally big dinoflagellates (Cullen et al., 2002) dominate. Reports on seasonal changes in bulk and size-fractionated biomass and community composition have increased largely over the last decades. Fewer descriptions of complete annual cycles of pelagic photosynthesis and primary production are available. In European waters, the studies we are aware of are mostly restricted to inshore
rini, 2002; Cermen ~o waters (Tillmann et al., 2000; Marty and Chiave et al., 2006; Gameiro et al., 2011). Photosynthesis-irradiance relationships or P-E curves are a convenient and widespread way of describing the physiological and acclimation response of phytoplankton assemblages to environmental changes (Sakshaug et al., 1997). Previous research has shown that photosynthetic parameters differ in the time-scales of response to new conditions established within the water column (Lewis et al., 1984). For instance, Geider (1993) noticed that

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n, R. Scharek / Estuarine, Coastal and Shelf Science 154 (2015) 255e263 X.A.G. Mora

stratification triggers changes in maximum chlorophyll normalized B ) but is usually less visible in the slope of photosynthetic rate (Pm the light-limited region (aB), claimed to be more constant than the former parameter (Behrenfeld et al., 2004; but see Cullen and Lewis, 1988). The frequent covariation of both photosynthetic parameters, extensively reviewed by Behrenfeld et al. (2004), has intrigued aquatic phycologists, but we still lack a complete mechanistic explanation for the so-called Ek-independent (i.e. strong B and aB) or E -dependent (no P B -aB positive correlation between Pm k m covariation) variability. Saturation irradiance (Ek) in turn frequently reflects current or recent light regimes (Tilstone et al., 2003).
n et al. (2009) have reviewed available studies on sizeButro fractionated phytoplankton biomass and primary production in nearshore and estuarine ecosystems of the Bay of Biscay, but we lack information about offshore waters. They concluded that bays and the outer areas of large estuaries usually exhibit marked phytoplankton peaks in spring, when the most favourable conditions for the development of phytoplankton are attained, since irradiances are high enough by that time and nutrients can become limiting in summer. However, on an annual basis riverine nutrient inputs are sufficient to sustain dominance by large (i.e. >2 mm) cells. In the southern Bay of Biscay open continental shelf, an annual cycle of photosynthetic carbon fixation by picophytoplankton and its relationship with community structure and growth rates were already described in Mor
an (2007), who concluded that approximately half of total primary production was accounted for by the smallest size-class. Here, we focus on the photosynthetic performance of large cells and the whole phytoplankton assemblage. The objectives of this study are 1) to describe and analyze seasonal variations in photosynthetic performance and primary production rates of total phytoplankton and the fraction >2 mm in relation to water-column properties, and 2) to explore the potential of intraannual variability for predicting the total amount of organic carbon entering the food web using chlorophyll and environmental variables.

polycarbonate filters (47 mm diameter). The filters were kept frozen at
20  C until analysis. Usually within 1 wk they were then placed in 90% acetone at 4  C for 24 h and the fluorescence of the extract was measured without acidification using a Perkin Elmer LB-50s spectrofluorometer (excitation at 440 nm, emission at 685 nm), periodically calibrated with pure chl a solution. Chl a in the picoplanktonic, nanoplanktonic, and microplanktonic size fractions was estimated as the amounts retained on the 0.2, 2 and 20 mm filters, respectively. Additionally, 100 ml chl a samples from the two depths of the P-E relationships determinations were taken and processed as detailed in Mor
an (2007). Samples for analyzing nitrite, nitrate, phosphate and silicate were immediately frozen and their concentrations determined in the laboratory with a Technicon autoanalyzer following standard procedures (Grasshoff et al., 1999).

2.2. Large phytoplankton community composition Samples for large (large nano-plus microplankton) phytoplankton community composition were taken monthly at 0, 30 and 75 m. 100 ml were collected in brown glass bottles, preserved with acid Lugol's solution and kept in the dark until analysis under an € hl (1958) technique. Diinverted microscope using the Utermo atoms, dinoflagellates, flagellates and ciliates were quantified and differentiated into size classes. Interpolation was used for deriving the abundance of the 4 broad taxonomical groups at the depth of the subsurface chlorophyll maximum when sampling depths were not coincident with those of the P-E experiments. Integrated abundances from 0 to 75 m were also calculated (cells m
2). Total phytoplankton includes the picoplanktonic groups analyzed by
n (2007). Some flow cytometry and described in detail in Mora small cells within the nanoplankton size-class (i.e. 4e8 mm in diameter) may have been missed by combining both counting methods but we believe that total cell number estimates were close to actual values.

2. Methods 2.3. Photosynthetic parameters 2.1. Environmental variables Physico-chemical variables were measured and biological samples were collected at a continental shelf station (43.7 N, 5.6 W, 110 m depth) from January to December 2003 as detailed in Mor
an (2007). This is the central station of the monthly RADIALES
n in the central Cantabrian Sea (southern Bay of transect off Xixo
de Rioja. Vertical profiles of temperBiscay) on board the RV ‘Jose ature and salinity were obtained with a SeaBird 25 CTD probe and photosynthetically active radiation (PAR, 400e700 nm) was measured with a Biospherical QSP-2200 spherical quantum sensor. After calculation of the vertical light extinction coefficients (Kd), optical depths (z) for the water samples taken for the photosynthetic parameters experiments were determined as Kd z, with z as the original depth. Actual daily surface irradiance (PAR) on the date of the monthly experiments (E0) was measured with a LI-192SA (LICOR) quantum sensor, ranging from 10.0 to 38.4 mol photons m
2 d
1. Additionally, climatological monthly values of PAR based on horizontal insolation averaged for 22 years (NASA) were used as the expected irradiance at the surface without cloud cover (E0 exp). These values were higher than the measured E0 with heavily overcast skies on three occasions, but otherwise there was a very good correspondence between both surface daily PAR estimates (r ¼ 0.99, p << 0.001, n ¼ 9). For chlorophyll a concentration (chl a), 100 ml samples taken at 8 discrete depths (0, 10, 20, 30, 40, 50, 75 and 100 m) were sequentially filtered through 20, 2 and 0.2 mm Millipore

Samples for the P-E experiments were collected at approx. 2 m
n, 2007), and the depth of the subsurface chl a maximum (Mora hereinafter referred to as ‘surface’ and ‘deep’. Full details of the experimental setup, irradiance measurements within the incubator
n (2007). The method used and sample processing are given in Mora for separating large (nano-plus microphytoplankton) from picophytoplankton carbon fixation involved specific P-E curve fitting for three fractions, total, >2 and <2 mm. Two different models were used to fit chl a-normalized hourly primary production rates, depending on the presence [Platt et al., 1980, Eq. (1)] or absence of photoinhibition [Webb et al., 1974, Eq. (2)]:

h   . ih . i exp
bB E PsB P B ¼ PsB 1
exp
aB E PsB

(1)

h  . i B B 1
exp
aB E Pm P B ¼ Pm

(2)

in which PB is the chl.a-normalized photosynthetic rate (mg C mg B is the maximum chl.a-normalized photosynthetic chl a
1 h
1), Pm B is calculated as [P B  (aB/(aB þ bB)  (bB/ rate (same units as PB; Pm m B B bB/aB (a þ b )] in Platt's et al. model, with PsB as the maximum chl.anormalized photosynthetic rate without photoinhibition), aB is the initial, light-limited slope [mg C mg chl a
1 h
1 (mmol photons m
2 s
1)
1], bB is the photoinhibition parameter (same units as aB), E is the experimental irradiance (mmol photons m
2 s
1) and Ek is B /aB). the saturation irradiance (same units as E and calculated as Pm


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257

2.4. Primary production Vertical distributions of chl a (total and large fractions) and PAR were used with the corresponding photosynthetic parameters for estimating volumetric primary production rates at the different depths of the water column. Linear interpolation of photosynthetic parameters was used between surface and deep values and extrapolation of the latter values down to 75 m. Finally, trapezoidal integrations were made for the upper 75 m of the water column rather than the euphotic zone in order to allow better assessments of the role of large phytoplankton group composition in measured rates. The euphotic zone depths during 2003 was always shallower than the depth of integration, ranging from 27 (April) to 74 m (August). 2.5. Statistics P-E model fitting was performed with the non-linear option of KaleidaGraph software (v. 3.0). All other statistics (correlation analyses, ordinary least-squares linear regressions, paired t-tests and polynomial fitting for better showing seasonal patterns) were performed with Statistica software (v. 7.1). 3. Results 3.1. Environmental conditions Detailed descriptions of environmental properties at the study
n, 2007). Briefly, temsite during 2003 are given elsewhere (Mora perature and salinity seasonal variations induced changes in the upper mixed layer depth, which extended down to the bottom (110 m) in December and was usually shallowest (<15 m) in summer while the euphotic layer remained more stable year-round (51 ± 17 m, mean and SD). From June through October, the water column was stratified as indicated by stratification index values >0.01 kg m
4 (approximately equivalent to >0.05  C m
1). Vertical distributions of nutrient concentrations tended to reveal a similar seasonal pattern, except that the development of the phytoplankton bloom in spring decreased phosphate and nitrate concentrations in the upper layers to annual minima from March
n, 2007 for nithrough summer (Table 1, see also Fig. 1D in Mora trate). Silicate concentrations (Table 2) in the upper layers showed similar seasonal patterns as phosphate and nitrate and concentrations and surface winter molar ratios of nitrate-nitrite to silicate concentrations were between 1 and 2. Total chlorophyll a concentrations in the euphotic layer ranged from 0.06 to 3.89 mg m
3, with subsurface maxima usually located between 20 and 40 m depth, except in June and July, when they were found slightly
n, 2007). As indicated by chl a deeper at 50 m (see Fig. 1C in Mora size fractionation, large phytoplankton (>2 mm) dominated the community year-round with only 3 months of picophytoplankton

Fig. 1. Monthly variation in the integrated (0e75 m) cell abundance of large phytoplankton groups (A) and their percent contribution (B).

dominance (August, October and November, Mor
an, 2007). Within the large fraction, the relative contribution of nanoplankton was generally much higher than that of microplankton (mean annual values of 40% ± 11% SD and 18% ± 18%, respectively), with dominance of microplankton only in March and April (~57%). During the rest of the year, microplankton contributions were generally <10%. The broad groups of large phytoplankton varied seasonally (Fig. 1), with a marked dominance of diatoms during the winterespring transition, exceeding 1.8  109 cells m
2. In April diatoms made up as much as 93% of total cell numbers of large phytoplankton integrated down to 75 m, while dropping to 2% in August and December. Contribution of other groups was less variable, with dinoflagellates slightly more abundant than other flagellates, mostly big nanoflagellates (10e20 mm) on an annual basis (36 vs. 28%). Both groups clearly dominated the community for the second half of the year, with ciliates (including potentially mixotrophic species) contribution close to 20% from July through

Table 1 Mean (±SE) annual values of photosynthetic parameters for small (<2 mm) and large (>2 mm) phytoplankton in surface and deep samples of the study site in 2003. Highlighted in bold are significant differences (p < 0.05, paired t-tests) between both size fractions. n ¼ 11e12. Surface

aB

[mg C mg chl a
1 h
1 (mmol photons m
2 s
1)
1] B Pm (mg C mg chl a
1 h
1) bB (same units as aB) Ek (mmol photons m
2 s
1) *

n ¼ 3.

Deep

Small

Large

0.018 (0.002)

0.026 (0.004)

Small 0.044 (0.008)

Large 0.023 (0.003)

5.90 (1.27)

4.94 (0.66)

4.93 (0.88)

2.30 (0.32)

435 (137)

236 (55)

0.0008* (0.0005) 129 (25)

0.0002* (0.0001) 122 (24)


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258

Table 2 Monthly integrated or averaged values (0e75 m) of phosphate and silicate (mmol L
1), total and large (>2 mm) chlorophyll (mg m
2) and primary production rates (mg C m
2 d
1) at the sampled station. Also shown are the relative contributions of large phytoplankton to total biomass as chlorophyll (BL/BT) and production rates (PL/PT) and the integrated primary production to chlorophyll ratios for both size fractions (PP/Chl, mg C mg chl
1 d
1). Date 2003

PO4

SiO2

Chltot

Chl>2 mm

PPtot

PP>2 mm

BL/BT

PL/PT

PP/Chltot

PP/Chl>2 mm

16 Jan 17 Feb 19 Mar 14 Apr 13 May 11 Jun 14 Jul 13 Aug 18 Sep 15 Oct 11 Nov 15 Dec Mean SE

0.33 0.13 0.10 0.07 0.04 0.13 0.12 0.14

1.74 2.77 0.87 1.01 0.67 0.64 0.59 0.52

27.9 43.1 25.0 197.7 22.1 28.8 31.7 23.6 39.8 25.2 37.8 35.8 44.9 14.0

14.7 23.4 20.5 181.1 15.1 15.6 17.6 8.6 24.4 9.8 16.2 21.1 30.7 13.7

214.8 228.9 304.7 891.8 510.9 131.5 198.1 518.5 656.0 485.4 363.5 150.0 391.1 67.6

123.9 121.3 122.9 412.82 229.0 104.9 85.6 201.4 629.0 135.4 148.5 172.4 207.3 45.8

0.53 0.54 0.82 0.92 0.68 0.54 0.55 0.37 0.61 0.39 0.43 0.59 0.58 0.05

0.60 0.47 0.46 0.46 0.45 0.80 0.43 0.39 0.98 0.28 0.41 1.15 0.57 0.08

7.7 5.3 12.2 4.5 23.1 4.6 6.3 22.0 16.5 19.3 9.6 4.2 11.3 2.1

8.4 5.2 6.0 2.3 15.2 6.7 4.9 23.3 25.7 13.7 9.2 8.2 10.7 2.1

*

*

*

0.32 0.03 0.85 0.21 0.07

1.82 0.67 2.04 1.21 0.23

Nutrient samples were not available for September.

November. The diatom to dinoflagellate cell abundance ratio varied accordingly from 0.03 in December to 22 in April. Values greater than 1 were observed from March through May and also in July.

3.2. Photosynthetic parameters The initial slope of the P-E curve (aB) ranged from 0.004 to 0.049 mg C mg chl a
1 h
1 (mmol photons m
2 s
1)
1, with almost the same ranges of variation for the total and large fraction (Fig. 2A, B). However, while surface and deep values of large phytoplankton were not significantly different year-round, aB of total phytoplankton was greater at depth from July through November, coincident with the period of summer stratification (Fig. 2B, paired ttest p ¼ 0.007, n ¼ 5). Maximum photosynthetic rates were also similarly variable for both size fractions (0.20e9.78 for the large and 0.13e10.68 mg C mg chl a
1 h
1 for the total), characterized by relative maxima in summer at the surface and minima in June deep samples (Fig. 2C, B values were consistently greater than at depth D). Surface Pm (paired t-tests, p ¼ 0.02 for total and <0.001 for >2 mm phytoplankton, n ¼ 12). Photoinhibition was detected only in summer (0.22e18.2$10
4 mg C mg chl a
1 h
1) with increasing values from June through August for both total and large phytoplankton, and 3fold higher values on average for large cells. Saturation irradiance of surface samples was seasonally more variable for total phytoplankton than for the large fraction, with higher values during summer higher irradiances (Fig. 2 F). Except in July (Fig. 2F), actual mean surface irradiances were generally much higher than Ek values, indicating year-round light-saturated photosynthesis in the upper layers. In contrast, mean irradiances at depth were always lower than the corresponding Ek values (Fig. 2E, F), although they were positively correlated for both large and total phytoplankton (r ¼ 0.74 and 0.80, p ¼ 0.002 and 0.006, respectively, n ¼ 12). B during Largely due to the different behaviour of aB and Pm summer stratification in bulk assemblages (Fig. 2B, D), no covariance was observed for total phytoplankton, neither for the entire dataset nor any depth subset. However, for large phytoplankton these photosynthetic parameters were positively correlated both in surface (r ¼ 0.77, p ¼ 0.003, n ¼ 12) and deep (r ¼ 0.66, p ¼ 0.02, n ¼ 12) samples, and with all data pooled (Fig. 3A). Large phytoplankton aB values were also significantly and negatively correlated with the absolute abundance of diatoms, with surface and deep data showing the same pattern (Fig. 3B).

B and E were negatively correlated with With all data pooled, Pm k optical depth for both total and large phytoplankton (Fig. 4B, C). While aB bore no significant correlations, the signs were opposite in the two groups, negative for cells >2 mm and positive for the total assemblage, due to the significant positive correlation when only
n, picophytoplankton data were considered (Fig. 4A, see also Mora 2007). Although the time-scale available for light acclimation differs for mixed and stratified waters, the relationships shown in Fig. 4 were essentially the same when the dataset was split in the two periods (data not shown).

3.3. Primary production As expected, volumetric primary production rates consistently decreased vertically (data not shown). Integrated (0e75 m) values (Table 2) ranged from 127 to 896 and 88e545 mg C m
2 d
1 for total and the large fraction, respectively. Both total areal biomass and productivity were significantly correlated with the contribution of large cells to the phytoplankton assemblage, measured as the number of cells >2 mm, and the importance of diatoms, either as absolute abundance or the diatom-to-dinoflagellate ratio (r ¼ 0.73e0.98, p < 0.008, n ¼ 12). Mean annual values of the integrated contribution of large phytoplankton to total biomass (BL/BT) and production (PL/PT) were virtually the same (paired t-test, p ¼ 0.93, n ¼ 12) but they showed a slightly different seasonality, with BL/BT ratios consistently higher than PL/PT from February to May (Table 2). While BL/ BT was significantly correlated with Kd (r ¼ 0.71, p ¼ 0.01, n ¼ 12), PL/PT increased with higher phosphate concentrations, both at the surface (r ¼ 0.85, p ¼ 0.001, n ¼ 11) and averaged for the upper 75 m (Table 2, r ¼ 0.76, p ¼ 0.007, n ¼ 11). Corresponding integrated primary production to chlorophyll ratios (PP/Chl) varied over one order of magnitude (2.3e23.3 mg C mg chl
1 d
1) and were similar for total and >2 mm phytoplankton (Table 2, paired t-test, p ¼ 0.69). PP/Chl>2 mm increased with stratification index (r ¼ 0.65, p ¼ 0.02, n ¼ 12) while PP/Chltot was significantly correlated with measured daily irradiance at the surface E0 (r ¼ 0.59, p ¼ 0.03, n ¼ 12). Variability in PP/ Chl ratios was thus seasonally predictable, especially for the large fraction, and when PP calculations considered the expected, climatological surface irradiance (E0 exp) rather than actual measurements (E0). This new variable (E0 exp PP/Chl ratio) corrected for the very low irradiances measured in June and especially in July 2003, due to overcast conditions (see Fig. 2E and F). Although for the rest of the year E0 values were virtually the same as E0 exp,


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259

B and E at the surface and deep waters of the sampled station for the fraction >2 mm (A, C, E) and the total Fig. 2. Monthly variation of the photosynthetic parameters aB, Pm k phytoplankton assemblage (B, D, F). In E and F mean values of irradiance measured at the two depths are also given (smaller symbols and dashed lines). The shaded area represents stratified conditions (i.e. stratification index >0.01 kg m
4). Error bars represent standard errors of the estimates.

12 mol photons m
2 d
1 measured in July were far below the expected value of 48 mol photons m
2 d
1 (similarly, 34 vs. 47 mol photons m
2 d
1 in June). E0 exp PP/Chl maxima were generally attained in summer while minima characterized winter and autumn (Fig. 5A). A strong positive relationship was found between its values and stratification index (Fig. 5B). Since PP/Chl ratios covaried positively with surface irradiance, when corrected by E0, yielding J, the water-column light utilization efficiency as defined by Falkowski (1981), variability of J values of total and large phytoplankton was similar (0.13e0.88 and 0.09e0.81 mg C mg chl
1 mol photons
1 m
2, respectively, Fig. 6) but only the latter showed a consistent seasonal pattern. J>2 mm was significantly correlated with mean phosphate concentrations in the 0e75 m depth interval used for calculations (Table 2, r ¼ 0.67, p ¼ 0.03, n ¼ 11).

4. Discussion 4.1. Photosynthetic performance of total and large phytoplankton Our results indicate that the photosynthetic behaviour of bulk phytoplankton assemblages (Fig. 2B, D, F) is a combination of the different strategies of the two size fractions examined, large (>2 mm, this work) and small (<2 mm, Mor
an, 2007). For instance, the overall higher values of aB at depth during summer stratifica
n, 2007) tion were largely attributable to picophytoplankton (Mora since large phytoplankton values were essentially undistinguishable at the two depths (Fig. 2A). Table 1 shows significantly higher B compared with the larger values of picophytoplanktonic aB and Pm fraction at depth. Higher values of both photosynthetic parameters in small phytoplankton compared with the large fraction were

260


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B and aB for the fraction >2 mm (surface and deep Fig. 3. A. Relationship between Pm sample symbols as in Fig. 2). Fitted OLS linear regression for pooled data: B ¼ 0.60 þ 124.53 aB, r2 ¼ 0.46, p < 0.001, n ¼ 24. B. Relationship between aB values Pm of phytoplankton >2 mm and the abundance of diatoms in the samples. Fitted OLS linear regression for pooled data: aB ¼ 0.044 e 0.006 þ log aB, r2 ¼ 0.29, p ¼ 0.008, n ¼ 23.

similar to other temperate coastal water surveys (Platt et al., 1983; Joint and Pomroy, 1986; Frenette et al., 1996), a feature different
ndez et al., from open-ocean, strongly oligotrophic waters (Ferna 2003). On an annual basis large phytoplankton maximum photosynthetic capacity covaried with the initial slope (Fig. 3A), thus showing ‘Ek-independent variability’ (Behrenfeld et al., 2004), although some acclimation of Ek was still observed especially on the vertical scale (Fig. 4C) rather than temporally (Fig. 2E). Our data thus suggest that Ek-independent variability can only be expected for large cells or in regions and periods in which phytoplankton is clearly dominated by the largest size-classes (Claustre et al., 1997;
n and Estrada, 2001). In Basterretxea and Arístegui, 1999; Mora their study of potential physiological mechanisms for this phenomenon, Behrenfeld et al. (2004) did not assess the specific role of cell size. Differential metabolic pathways in photosynthetic prokaryotes (<2 mm) and eukaryotes (found in all size classes) responsible for the balance between reductants and ATP generation (Behrenfeld et al., 2004) cannot be excluded. Although the reason for this observation remains elusive, larger cells are more prone to B in the same direction in response to a changing modify aB and Pm B and aB in light environment than small cells, which changed Pm opposite directions when the water-column of the study site was
n, 2007). strongly stratified (Mora The vertical photoacclimation of the maximum photosynthetic rate was much more robust and widespread than the other

B and E and the Fig. 4. Relationships between the photosynthetic parameters aB, Pm k optical depth for pooled data of the fraction >2 mm and the total phytoplankton assemblage. Continuous fitted lines represent significant correlations and include also
n (2007) (thick grey, the relationships for the small fraction described in detail in Mora total; thick black, >2 mm; thin black, <2 mm) correlation coefficients: 0.65** (aB B total),
0.66*** (P B >2 mm),
0.56** (E total),
0.44* <2 mm),
0.60** (Pm k m (Ek > 2 mm),
0.5* (Ek < 2 mm)*p < 0.05, **p < 0.01, ***p > 0.001, n ¼ 24. Dashed lines are non significant correlations showed only for reference.

photosynthetic parameters (Fig. 4). Similar to winterespring ob
n and Estrada, 2005), servations in the NW Mediterranean (Mora we failed to find any significant pattern of the initial P-E slope on an annual basis for either large or total phytoplankton (Fig. 4A), contrary to previous work reporting increasing aB values deeper in the water column (Dower and Lucas, 1993; Basterretxea and Arístegui,
n et al., 2001) or, less frequently, decreasing (Mora
n and 2000; Mora B rather Estrada, 2001). Surface assemblages generally lie closer to Pm than spend too much time on the light-limited region and Ek was almost always well below daily irradiance (Fig. 2E, F) as found ^ te
and Platt, 1983). On the contrary, Ek values at elsewhere (e.g. Co


n, R. Scharek / Estuarine, Coastal and Shelf Science 154 (2015) 255e263 X.A.G. Mora

Fig. 5. A. Monthly variation of the PP/Chl ratios for the fraction >2 mm and total phytoplankton using expected surface irradiances (see the text for details). Fitted line for the fraction >2 mm: E0 exp PP/Chl>2 mm ratio ¼ 15.81e0.50day þ 5.93  10
3 day2 e 2.15  10
5 day3 þ 2.38  10
8 day
4, r2 ¼ 0.71, n ¼ 12. B. Relationship between the >2 mm values shown in (A) and the density stratification index (SI) of the water column. Fitted OLS linear regression: E0 exp PP/Chl>2 mm ratio ¼ 5.80 þ 419.1 SI, r2 ¼ 0.73, p ¼ 0.0002, n ¼ 12.

depth were considerably higher than measured irradiances indicating light-limited conditions although probably mixing made cells spend some time at shallower depths. However, deep Ek values significantly followed changes in the light field (Fig. 2E,F), suggesting that there both large and small phytoplankton communities were able to adjust Ek in a similar fashion, in contrast to the aB parameter (cf. Fig. 4A and C). In any case, the slope of the linear B and aB (125 mmol photons m
2 s
1, Fig. 3A) regression between Pm summarizes the year-round Ekdindependent variability of large phytoplankton. Although differences between surface and deep Ek values were also detected during stratification in the >2 mm fraction (Fig. 2E), they were much larger when the total assemblage was considered (Fig. 2F). All in all, large phytoplankton showed little photoacclimation (MacIntyre et al., 2002) compared with
n, 2007) even in stratified conditions, as picophytoplankton (Mora also noted by Uitz et al. (2008). 4.2. Primary production rates Since the introduction of the 14C technique by SteemannNielsen (1952), thousands of measurements of pelagic primary production have been performed. However, the seasonal patterns, accounting for a large fraction of its variability in mid-to high latitude ecosystems, are only well known for a few sites worldwide. Our total primary production rates were generally low to moderate in 2003 with the exception of April (Table 2). Although we observed

261

a marked diatom bloom in that month (Fig. 1), total PP did not even reach 1 g C m
2 d
1, far from the values measured during the winterespring blooms in more productive areas nearby such as the ~ o et al., 2006; OspinaGalician Rías (e.g. Varela et al., 2005; Cermen Alvarez et al., 2014) or even in the NW Mediterranean (Estrada,
n and Estrada, 2005) and refs. therein), traditionally 1996; Mora regarded as a more oligotrophic ecosystem. However, our total annual primary production estimate of 139 g C m
2 y
1 is well in agreement with 37 year hindcasted values along the Bay of Biscay continental shelf (Huret et al., 2013). With the limitation of just one year of sampling, this primary production rate would place the mesotrophic southern Bay of Biscay (Cantabrian Sea) continental shelf closer to oligotrophic conditions (Nixon, 1995; Cullen et al., 2002), especially if we take also into account the measurements made at a mid-shelf station off Cuideiru, located approx. 50 km
n, which for the 1992e2007 period were eastwards of Xixo considerably lower than our values mean 83 mg C m
2 d
1, (Bode et al., 2011). We cannot discard substantially higher annual primary production rates at the study site in other years because of the anomalously low SiO2 concentrations measured in 2003 (e.g. in 2004 mean concentrations in the 0e75 m layer were more than B values were twice as high for double). Although both aB and Pm picophytoplankton than for the large fraction at depth (Table 1),
n, 2007), especially during stratification (cf. Figs. 2 and 4 in Mora with a mean annual value of 75 g C m
2 y
1, production by large phytoplankton slightly dominated over that of small cells in the upper 75 m (Table 2). Phytoplankton standing stocks are frequently used as proxies of productivity (e.g. Falkowski et al., 1998; Vollenweider et al., 1998;
n both variables covaried Boyce et al., 2010), and indeed off Xixo positively both for the total and the large fraction, although the correlation was only significant for total phytoplankton (r ¼ 0.70, p ¼ 0.011, n ¼ 12). However, environmental controls differed when a complete seasonal cycle was considered. For instance, although sharing a mean annual value of ~58% (Table 2), it was the variable large cells relative production rather than biomass that correlated positively with inorganic nutrient concentrations (significantly only for phosphate). Furthermore, dissimilar patterns of BL/BT and PL/PT were identified for spring in comparison to the rest of the year, which was probably related to the different specific composition of large phytoplankton assemblages. From March through May, BL/BT values >0.60 (Table 2) due to annual maximum abundances of diatoms (Fig 1), were characterized by a constant, rather low PL/PT value of 0.46. These values might have been greater had we sampled algal blooms at the initial rather than the decay phases probably characterizing our samples. However, this pattern may also be a consequence of the decrease in the light utilization efficiency parameter (aB) with increasing diatom contribution found for all monthly data (Fig. 3B). On the other hand, PL/PT peaks in June, September and December corresponded to very different assemblage conditions. While the September peak in absolute and relative diatom abundance (Fig. 1) could suggest that the sporadic autumn blooms were more efficient than spring blooms (Figs. 5A and 6), June and December samples shared a virtually equal contribution of flagellates and dinoflagellates (Fig. 1), with the highest abundances of the latter group recorded in 2003 (1.2$109 cells m
2). The lowest annual contribution of diatoms found in December suggests that this group does not need to be present for large cells dominance of primary production (Tremblay and Legendre, 1994). Water-column assimilation numbers or PP/chl ratios varied from 2 to 26 and from 5 to 22 g C g chl
1 d
1 for large and total phytoplankton, respectively (Table 2), well within reported ranges (Falkowski, 1981) and seasonality similar to other temperate regions, with minima in autumn and winter and maxima in early

262


n, R. Scharek / Estuarine, Coastal and Shelf Science 154 (2015) 255e263 X.A.G. Mora

Fig. 6. Monthly variation of water-column light utilization efficiency for total and large phytoplankton. Fitted line for the fraction >2 mm: 0.955e0.021day þ 1.81  10
4 day2 e 5.47  10
7 day3 þ 5.66  10
10 day
4 r2 ¼ 0.84, n ¼ 12.

summer (Tillmann et al., 2000; Shaw and Purdie, 2001; Jouenne et al., 2007). When the expected rather than measured irradiance (i.e. without cloud cover) was used, predictability of PP/chl>2 was very strong (Fig 5A), which would allow us to estimate monthly areal production rates of large phytoplankton from just biomass measurements if future investigations confirm the marked seasonal pattern found. Vertical changes in production rates were much more marked than in biomass, resulting in negative exponential decreases of PP/Chl ratios vs. depth (from
0.05 to
0.16 m
1) that became steeper with higher Kd values (r ¼ 0.98, p << 0.01, n ¼ 12), thus illustrating the major role played by underwater irradiance. This can help explain the perhaps counterintuitive fact that summer maximum PP/chl>2 mm ratios were 10-fold larger than April values (Table 2) where the highest monthly PP was measured. A shading effect due to accumulated biomass and an approx. 4-fold decrease in aB (Fig. 3B) may be behind the decrease in PP/Chl>2 mm observed in April. Integrated biomass was however approx. twice more variable than assimilation numbers, thus holding the general covariance between standing stocks and productivity. Increasing PP/chl values as stratification progressed such as we show in Fig. 5B had already been found in temporally more restricted surveys
n and Estrada, 2005). This pattern could exacerbate in the (Mora coming decades (Capotondi et al., 2012). Water-column light utilization efficiency values (J) fit within the range given by Falkowski and Raven (1997). Similarly to PP/Chl ratios (Fig. 5A), they were also seasonally variable for large cells, although maxima were lagged here towards the end of the year: winter-early spring values (JaneApr mean 0.30 g C g chl
1 mol photons
1 m2) were considerably lower than late summereautumn ones (SepeDec mean 0.69 g C g chl
1 mol photons
1 m2, Fig. 6). Predictability of these productivity indices was higher for large phytoplankton than for the total (Figs. 5A and 6), partially due to the different photosynthetic behaviour of picophytoplankton on the vertical scale (cf. Figs. 2 and 4 in Mor
an, 2007). Contrary to Hashimoto and Shiomoto (2002), higher J values were usually found for picophytoplankton (data not shown) than for large phytoplankton. Seasonality of picophytoplankton J displayed no clear temporal pattern and bore no significant relationship with any of the explored variables, with the most conspicuous difference being that its maximum values (1.1e1.5 g C g chl
1 mol photons m2) were attained from March through May, virtually coincident with the period of large phytoplankton J minima. The well-known seasonal shift in the relative importance of large vs. small phytoplankton biomass (Winder and Cloern, 2010) resulted in the

opposite in terms of water-column light utilization efficiency during the spring diatom-dominated phytoplankton bloom, likely related with the different response of phytoplankton size-fractions and functional groups to nutrient availability (Kiørboe, 1993; Raven, 1998). Diatoms are the only phytoplankters that need silicate as an essential nutrient. During the spring bloom in April nitrate and silicate concentrations were close or below the halfsaturation constant values reported for coastal diatom species (Egge and Aksnes, 1992; Lomas and Glibert, 2000), which implies that the diatoms were not growing at their maximum rate (Falkowski et al. 1991). It should be noted that the correlation between average inorganic nutrient concentrations and J, which was significantly positive with phosphate for large cells, disappeared for picophytoplankton. Although mean concentration of nutrients in the water column are a bulk estimate of nutrient availability at every depth, especially in strongly stratified conditions such as those found between June and October, serious nutrient limitation in the upper mixed layer cannot be overcome by high concentrations below the nutricline. Similarly to PP/Chl ratios, the vertical profiles of phytoplankton abundance (as Chl) could have played a role in the seasonal changes of J, but we should take into consideration that seasonality of PP/Chl ratios and J was completely different for the last third of the year (September to December, cf. Figs. 5 and 6). Lower individual chlorophyll content in the upper layers in summer is a direct response to higher irradiance but both variables are in the denominator of J, thus partially cancelling out each other. Altogether, we suggest that nutrient availability played an important role in determining the seasonal changes in the water-column light utilization efficiency of large phytoplankton. Analysis of a decade of data from the station analyzed here plus the other two stations routinely sampled on the continental shelf
n give little support to the existence of bimodal distribuoff Xixo tions of phytoplankton biomass at this site contrary to other nearby areas (Bode et al., 2011; O'Brien et al., 2012). The lack of a persistent autumn bloom does not mean that PP is only high during the winterespring transition. Altogether, our results suggest that on the Southern Bay of Biscay continental shelf the diatom spring bloom, although responsible for the highest monthly PP value measured in 2003, does not necessarily translate into sustained high PP values. Rather, PP peaks could be observed sporadically during other low biomass months where light utilization efficiency and production to chlorophyll ratios became maximal. Indeed, bimodal distributions of zooplankton abundance and biomass at the study site (O'Brien et al., 2013) suggest tight grazing responses to both the persistent spring PP peaks and the more sporadic ones found in autumn. In summary, the annual cycle of planktonic photosynthetic parameters and primary production in the Southern Bay of Biscay continental shelf showed a widely different response and season
n, 2007) and large ality for small (i.e. picophytoplankton, Mora (nano-plus microplankton, this study) cells. Our analysis shows that monthly variations in primary production at this mesotrophic site are not necessarily well captured by changes in chlorophyll. Total primary production will be better modelled after splitting the phytoplankton assemblage into size classes, with the large fraction apparently more responsive to seasonally predictable factors.

Acknowledgements
Lamas for their help with P-E We thank A. Calvo-Díaz and A. incubations and large phytoplankton analysis. Inorganic nutrients were analyzed by N. Gonz
alez and C. Carballo. This work was financially supported by the Spanish research grant VARIPLACA


n, R. Scharek / Estuarine, Coastal and Shelf Science 154 (2015) 255e263 X.A.G. Mora

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